Optical transmission systems for digital transmission of wavelength multiplexed signals over medium and long haul distances can experience several kinds of cross-talk effects. One of these effects is Cross-Phase Modulation. This arises at high signal power levels at which non-linearity effects begin to assume significant proportions and limit system reach. Cross-Phase Modulation arises when the modulation of the optical power in one of the signals being transmitted is large enough to produce a significant corresponding modulation of the refractive index of the transmission medium in which that power is propagating. This index modulation give rise to phase modulation of all the signals being transmitted. Phase modulation of the signal that is producing the index modulation is Self-Phase Modulation (SPM), while phase modulation of all the other signals is Cross-Phase Modulation, XPM.
In a transmission system that uses direct detection the presence of XPM at the detector presents no particular problem because a direct detector is not sensitive to variations in phase of the received signal. XPM is however a problem, even in a transmission system using direct detection, if the transmission medium exhibits dispersion. In the presence of such dispersion, as is for instance exhibited by conventional optical fibre waveguide transmission media handling wavelength multiplexed traffic, the phase modulation produced by XPM is progressively converted into amplitude modulation. Hence the presence of XPM is liable to lead to partial closure of the transmission eye diagram.